As someone who works on a SaaS product, one of our biggest costs is our stable API surface. Internal APIs are essentially free, but for a public API we have to:

- Implement a conversion layer from our internal representation so we can keep it stable.

- Complicate all further feature work because we have to consider how it will affect existing customers of the API.

- Write and maintain documentation for the API.

- Keep the API working even after we no longer use it.

- Maintain multiple versions of the API in parallel.

- Make sure our error messages make sense to people not familiar with our internal systems.

- Be more careful with validation - for our internal APIs it's not the end of the world if a bad request results in a 500 rather than a 400, but it matters a lot for public APIs.

- Be more careful with rate limiting and other defenses against API misuse.

And this is to name just a few. A requirement that everyone expose a public API is pointless if it doesn't include a stability guarantee, and overly burdensome if it does.